All materials that depend on homogeneous nucleation for crystal growth supercool. Thus, the material can be slowly raised to its melting point (Te) and then the temperature of the melt can be lowered to a value Ts before freezing takes place. A measure of the supercooling is .DELTA.T = (Te - Ts )/Te. This value ranges form 0.1 to 0.3 for many elements and compounds while the value increases for alloys and heavily doped simiconductors. As the value increases it becomes more and more difficult to grow homogeneous, highly perfect, single crystals. Regardless of the method of crystal growing, theory shows that the growth rate (f) should be slow and the temperature gradient (g) across the solid-liquid interface should be as large as possible so that the ratio g/f (.degree.C sec/cm.sup.2) is a large number. When the degree of supercooling increases (.DELTA.T&gt;0.5), analysis shows that g/f must be greater than 5.times.10.sup.6 .degree.C sec/cm.sup.2. This means that for a slow growth rate of 1 inch per day, the thermal gradient must exceed 140.degree.C cm.sup..sup.-1.
There exists a large class of ternary compounds having unusual semiconducting and optical properties that have not been investigated in any detail. These compounds can be represented by the formulas M.sub.3 AB.sub.3, M.sub.4 A.sub.2 B.sub.5 and MAB.sub.2, wherein A is P, As or Sb; B is S, Se or Te; and M is Na, K, Li, Ba, Ca, Mg, Be, Zr, Mn, Zn, Ce, Fe, Co, Ni, Cd, Pb, Sn, Cu, Ag, Au or Pt. Many of these compounds are natural minerals such as wolfsbergite (CuSbS.sub.2), and the like. Of particular interest are proustite (Ag.sub.3 AsS.sub.3), pyragyrite (Ag.sub.3 SbS.sub.3) and smithite (AgAsS.sub.2). These latter compounds are uniaxial (3m), have large birefringence (.about.0.2) and are transparent in the near infrared (1-15.mu.) where high power gas lasers are available. These compounds have large non-linear coefficients and they have made efficient phase-matched harmonic generators and parametric oscillators. However, crystals are very difficult to grow from these materials because they exhibit large supercooling. For example, proustite melts at 480.degree.C and can undercool by 150.degree.C. This yields a .DELTA.T of about 0.7 which means that g/f must exceed about 5.times.10.sup.5 .degree.C sec/cm.sup.2.
It is an object of this invention, therefore, to provide a method of crystal growth that is particularly applicable to precursor compounds which exhibit large supercooling.
Another object of the invention is to provide apparatus for use in conducting the method of crystal growth.